Body Armor is a variation on INVULNERABILITY that wears down as it takes repeated damage above its rated value. This reflects comic characters who start out with nice armor or a shield or bulletproof vest or whatever and then, after a battle or two, it's pretty much shredded and not providing much protection.
In this case BODY ARMOR starts out just like INVULNERABILITY. But, its effectiveness is reduced by 10% (rounded up) of the difference in damage and its rating, every time the armor's current rated damage is exceeded. So, if I have 20 points of BODY ARMOR and I am hit by a 15 damage attack, I take no damage and I am ready for the next one. But, if I take a 25 damage attack, then only 5 points of it get past the armor, but my armor has been damaged and next time it will only absorb 19 points of damage (it's rating was exceeded by 5 point and 10% of 5 rounded up is a 1 point reduction in the armor). And so on until it is gone.
This means that 5 points of body armor will be gone pretty quickly. (The 2 and 5 point BODY ARMOR versions are mostly there to allow folks to fine-tune their combo attributes, e.g. combine to make 7, 15, etc. point BODY ARMOR combos.). But, 30 points is likely to last through most of a mission, since it would take many attacks to get rid of it and those attacks would have to do pretty high damage early on.
The code supports FFX removability and is compatible with the INVULNERABLE and PHYSICAL RESISTANCE attributes. For characters with both BODY ARMOR and one of those attributes, the damage comes off of the BODY ARMOR first to reflect it being the "outer shell" of protection, before any inborn toughness is hit. So a character with 10 points of INVULNERABILITY and 10 points of BODY ARMOR will take no damage from a 15 point attack, but his BODY ARMOR rating would drop to 9 points next time, since the 15 points of damage exceeded its rating.